A Tao of Regular Expressions

Steve Mansour
sman@scruznet.com
Revised: June 5, 1999 (copied by jm /at/ jmason.org from http://www.scruz.net/%7esman/regexp.htm, after the original disappeared! )

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C O N T E N T S

What Are Regular Expressions
Examples
   Simple
   Medium (Strange Incantations)
   Hard (Magical Hieroglyphics)
Regular Expressions In Various Tools

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What Are Regular Expressions

A regular expression is a formula for matching strings that follow some pattern. Many people are afraid to use them because they can look confusing and complicated. Unfortunately, nothing in this write up can change that. However, I have found that with a bit of practice, it's pretty easy to write these complicated expressions. Plus, once you get the hang of them, you can reduce hours of laborious and error-prone text editing down to minutes or seconds. Regular expressions are supported by many text editors, class libraries such as Rogue Wave's Tools.h++, scripting tools such as awk, grep, sed, and increasingly in interactive development environments such as Microsoft's Visual C++.

Regular expressions usage is explained by examples in the sections that follow. Most examples are presented as vi substitution commands or as grep file search commands, but they are representative examples and the concepts can be applied in the use of tools such as sed, awk, perl and other programs that support regular expressions. Have a look at Regular Expressions In Various Tools for examples of regular expression usage in other tools. A short explanation of vi's substitution command and syntax is provided at the end of this document.

Regular Expression Basics

Regular expressions are made up of normal characters and metacharacters. Normal characters include upper and lower case letters and digits. The metacharacters have special meanings and are described in detail below.

In the simplest case, a regular expression looks like a standard search string. For example, the regular expression "testing" contains no metacharacters. It will match "testing" and "123testing" but it will not match "Testing".

To really make good use of regular expressions it is critical to understand metacharacters. The table below lists metacharacters and a short explanation of their meaning.
 

Metacharacter		Description
.		Matches any single character. For example the regular expression r.t would match the strings rat, rut, r t, but not root.
$		Matches the end of a line. For example, the regular expression weasel$ would match the end of the string "He's a weasel" but not the string "They are a bunch of weasels."
^		Matches the beginning of a line. For example, the regular expression ^When in would match the beginning of the string "When in the course of human events" but would not match "What and When in the" .
*		Matches zero or more occurences of the character immediately preceding. For example, the regular expression .* means match any number of any characters.
\		This is the quoting character, use it to treat the following character as an ordinary character. For example, \$ is used to match the dollar sign character ($) rather than the end of a line. Similarly, the expression \. is used to match the period character rather than any single character.
[ ]  [c1-c2] [^c1-c2]		Matches any one of the characters between the brackets. For example, the regular expression r[aou]t matches rat, rot, and rut, but not ret. Ranges of characters can specified by using a hyphen. For example, the regular expression [0-9] means match any digit. Multiple ranges can be specified as well. The regular expression [A-Za-z] means match any upper or lower case letter. To match any character except those in the range, the complement range, use the caret as the first character after the opening bracket. For example, the expression [^269A-Z] will match any characters except 2, 6, 9, and upper case letters.
\< \>		Matches the beginning (\<) or end (\>) or a word. For example, \<the matches on "the" in the string "for the wise" but does not match "the" in "otherwise". NOTE: this metacharacter is not supported by all applications.
\( \)		Treat the expression between \( and \) as a group. Also, saves the characters matched by the expression into temporary holding areas. Up to nine pattern matches can be saved in a single regular expression. They can be referenced as \1 through \9.
|		Or two conditions together. For example (him|her) matches the line "it belongs to him" and matches the line "it belongs to her" but does not match the line "it belongs to them." NOTE: this metacharacter is not supported by all applications.
+		Matches one or more occurences of the character or regular expression immediately preceding. For example, the regular expression 9+ matches 9, 99, 999. NOTE: this metacharacter is not supported by all applications.
?		Matches 0 or 1 occurence of the character or regular expression immediately preceding.NOTE: this metacharacter is not supported by all applications.
\{ i \} \{ i , j \}		Match a specific number of instances or instances within a range of the preceding character. For example, the expression A[0-9]\{3\} will match "A" followed by exactly 3 digits. That is, it will match A123 but not A1234. The expression [0-9]\{4,6\} any sequence of 4, 5, or 6 digits. NOTE: this metacharacter is not supported by all applications.

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The simplest metacharacter is the dot. It matches any one character (excluding the newline character). Consider a file named test.txt consisting of the following lines:

he is a rat
he is in a rut
the food is Rotten
I like root beer

We can use grep to test our regular expressions. Grep uses the regular expression we supply and tries to match it to every line of the file. It prints all lines where the regular expression matches at least one sequence of characters on a line. The command

grep r.t test.txt

searches for the regular expression r.t in each line of test.txt and prints the matching lines. The regular expression r.t matches an r followed by any character followed by a t. It will match rat and rut. It does not match the Rot in Rotten because regular expressions are case sensitive. To match both the upper and lower the square brackets (character range metacharacters) can be used. The regular expression [Rr] matches either Ror r. So, to match an upper or lower case r followed by any character followed by the character t the regular expression [Rr].t will do the trick.

To match characters at the beginning of a line use the circumflex character (sometimes called a caret). For example, to find the lines containing the word "he" at the beginning of each line in the file test.txt you might first think the use the simple expression he. However, this would match the in the third line. The regular expression ^he only matches the h at the beginning of a line.

Sometimes it is easier to indicate something what should not be matched rather than all the cases that should be matched. When the circumflex is the first character between the square brackets it means to match any character which is not in the range. For example, to match he when it is not preceded by t or s, the following regular expression can be used: [^st]he.

Several character ranges can be specified between the square brackets. For example, the regular expression [A-Za-z] matches any letter in the alphabet, upper or lower case. The regular expression [A-Za-z][A-Za-z]* matches a letter followed by zero or more letters. We can use the + metacharacter to do the same thing. That is, the regular expression [A-Za-z]+ means the same thing as [A-Za-z][A-Za-z]*. Note that the + metacharacter is not supported by all programs that have regular expressions. See Regular Expressions Syntax Support for more details.

To specify the number of occurrences matched, use the braces (they must be escaped with a backslash). As an example, to match all instances of 100 and 1000 but not 10 or 10000 use the following: 10\{2,3\}. This regular expression matches a the digit 1 followed by either 2 or 3 0's. A useful variation is to omit the second number. For example, the regular expression 0\{3,\} will match 3 or more successive 0's.

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